In the production of reduced pellets such as reduced iron pellets by the grate-kiln system for firing through the use of a facility comprising a pretreatment furnace composed of a drying zone, a preheating zone and a hardening zone and provided with a travelling grate, and a rotary kiln, the following methods are known as methods of feeding green pellets onto said grate of pretreatment furnace, namely:
A. A method comprising the steps of sieving produced green pellets to predetermined grain sizes, measuring continuously the weight of sieved green pellets by a weighing machine while said green pellets are on the way to said grate, feeding forward a signal indicative of the measured value to said grate, and changing the travelling speed of said grate after the lapse of a predetermined time, i.e., increasing the travelling speed of the grate when the measured value is larger than a standard value and decreasing the travelling speed of the grate when the measured value is smaller than the standard value. PA1 B. Another method comprising the steps of storing temporarily green pellets, produced and sieved to predetermined grain sizes in the same manner as in (a) mentioned above, in a hopper provided immediately before the grate, feeding then green pellets from the lower part of the hopper onto the grate, continuously measuring the level of green pellets in said hopper, decreasing the travelling speed of the grate when the level is lower than a standard level, and decreasing the supply of material to the pelletizer when the level is higher than the standard level. PA1 C. Still another method comprising the steps of feeding all of the green pellets, produced and sieved to predetermined grain sizes in the same manner as in (a) mentioned above, onto the grate, and changing the travelling speed of the grate in response to variations in the thickness of the layer of green pellets on the grate, i.e., increasing the travelling speed of the grate when the layer of the green pellets is thick, and decreasing the travelling speed of the grate when the layer of the green pellets is thin.
However, in the above-mentioned methods (a), (b) and (c), the travelling speed of the grate changes so frequently that the time required for drying, preheating and hardening of green pellets in the pretreatment furnace varies to a great extent, disadvantageously resulting in complicated control of the pretreatment furnace and in unstable quality of reduced pellets obtained in the rotary kiln as well. Another disadvantage is observed in the case of the method (b) -- green pellets stored in the hopper are liable to be crushed in the hopper because of their low crushing strength.
In view of the foregoing, although a method is required which feeds constant quantity of green pellets onto the grate of the pretreatment furnace in the production of reduced pellets by the grate-kiln system for firing, whereby the necessity for changing the travelling speed of the grate is avoided, such method has not so far been proposed.